EnumProperty fill dynamically using callback
Since Blender 2.79 you can do the following.

###############################
## Portal plane
###############################

def is_wowgroup_enabled(self, obj):
    return obj.WowWMOGroup.Enabled


class WowPortalPlanePanel(bpy.types.Panel):
    bl_space_type = "PROPERTIES"
    bl_region_type = "WINDOW"
    bl_context = "object"
    bl_label = "Wow Portal Plane"
    bl_options = {'DEFAULT_CLOSED'}

    def draw_header(self, context):
        layout = self.layout
        self.layout.prop(context.object.WowPortalPlane, "Enabled")

    def draw(self, context):
        layout = self.layout
        row = layout.row()
        layout.enabled = context.object.WowPortalPlane.Enabled
        self.layout.prop(context.object.WowPortalPlane, "First")
        self.layout.prop(context.object.WowPortalPlane, "Second")     

    @classmethod
    def poll(cls, context):
        return (context.object is not None and context.object.data is not None and isinstance(context.object.data,bpy.types.Mesh))


class WowPortalPlanePropertyGroup(bpy.types.PropertyGroup):
    Enabled = bpy.props.BoolProperty(name="", description="Enable wow WMO group properties")
    First = bpy.props.PointerProperty(type=bpy.types.Object, poll=is_wowgroup_enabled, name="First group", description="First group")
    Second = bpy.props.PointerProperty(type=bpy.types.Object, poll=is_wowgroup_enabled, name="Second group", description="Second group")
    PortalID = bpy.props.IntProperty(name="Portal's ID", description="Portal ID")


def RegisterWowPortalPlaneProperties():
    bpy.types.Object.WowPortalPlane = bpy.props.PointerProperty(type=WowPortalPlanePropertyGroup)


def UnregisterWowPortalPlaneProperties():
    bpy.types.Object.WowPortalPlane = None